depends on what cpu block your planning to use, ideally go all nickel or copper but dont mix the two, i've had a look at a competitor and they have the actel/copper vega 56/64 block in stock at £94.99, i did check ek but they have no stock of any vega stuff, best bet is to shop aroud and if you have no luck then ebay will be your next port of call, just remember keep the blocks all the same in your loop and if buying second hand be sure to give all your blocks a good clean to remove all gunk and resedue.
one more thing the vega 56 and 64 cards use the same pcb layout so the waterblock will be 56 and 64 compatible and the back plates will be compatible as well
hope this helps